Remove the door from its hinges by tapping a screwdriver up through the bottom of the hinge to drive the pin out of the hinge. Lay the door on a pair of sawhorses to make the bottom panel easily accessible.
Scrape any old glue from the joints on either side of the bottom door rail with a razor scraper or sharp wood chisel. The rail is the bottom horizontal piece and is typically joined to the two outside vertical pieces, known as stiles. Scrape any old glue from the joint along the top of the rail as well.
Tap along the bottom of the door with a rubber mallet or hammer and woodblock to move the rail up into its original position with the bottom of the rail flush with the ends of the two side stiles. Tap the stiles in toward the center of the door to their original position to check the fit.
Tap a chisel or small flat pry bar into the joints at either side of the bottom rail and pry out slightly to access the tenon, or tongue, at each side of the rail. Apply wood glue to both faces of this tongue on either side of the rail. Tap the rails back into position.
Apply two bar clamps the width of the door, pressing against the outside edges of the stiles. Use folded cardboard or wood shims to pad the clamps where they touch the door. Tighten the clamps to hold the stiles firmly against the sides of the rail. Leave the clamps in place for at least three to four hours, overnight if possible, to allow the glue to harden.
Remove the clamps and hang the door back on its hinges. Apply a coat of polyurethane to the seams along both sides of the rail to seal it and prevent future problems.